This invention relates to a technique for detecting a surface height of a substrate and, more particularly, to an apparatus for detecting a surface height of a photosensitive substrate used in an exposure apparatus.
In an exposure apparatus used for manufacturing a semiconductor device or a liquid crystal display device, a pattern formed on a mask is transferred onto a photosensitive substrate under image-forming conditions that must be satisfactory. A current exposure apparatus typically uses an auto-focus mechanism to measure the height of the photosensitive substrate, that is, the position of the surface of the photosensitive substrate along the optical axis direction of the projection optical system. The stage on which the photosensitive substrate is loaded is moved by a driving system along the optical axis of the projection optical system to bring the surface of the photosensitive substrate into a focal point of the projection optical system. A known auto-focus mechanism emits a slit-like light beam (referred to as a slit beam) from an oblique direction with respect to the photosensitive substrate and detects the light beam reflected from the surface of the photosensitive substrate. When the surface height of the photosensitive substrate changes, the reflecting direction of the light beam from the surface of the photosensitive substrate also changes. The auto-focus mechanism utilizes this principle to detect the height of the photosensitive substrate surface.
FIG. 6 illustrates the reflecting state of a slit-like detection beam (slit beam) emitted by the auto-focus mechanism. The detection beam is guided to a glass plate 100, which is used for fabricating a liquid crystal display device. Because the glass plate 100 is transparent, a portion of the incident light beam is reflected by the top surface of the glass plate 100, which becomes a first reflected light beam R1. Another portion of the incident light beam passes through the top surface and is reflected by the bottom of the glass plate 100, which becomes a second reflected light beam R2. To detect the Z position (height) of the glass plate 100, the first reflected light beam R1 from the top surface must be sufficiently separated from the second reflected light beam R2 from the bottom surface. In other words, only the first light beam R1 reflected from the top surface must be accurately detected. To this end, the width of the slit beam is narrowed so that the second reflected light beam R2 from the bottom surface is not mixed with the first reflected light beam R1 from the top surface of the glass plate 100.
In recent years, various substrates with different thicknesses have been used in an exposure apparatus. If a substrate is thin, then a third light beam R3, which is reflected from the bottom surface of the thin substrate indicated by the dashed line, gets closer to the first light beam R1, which is reflected from the top surface thereof, as is illustrated by the dashed arrow. In this situation, the width of the slit beam must be further narrowed in order to completely separate the first light beam R1 reflected from the top surface from the third light beam R3 reflected from the bottom surface.
By narrowing the width of the slit beam, the first reflected light beam R1 from the top surface can be reliably separated from the second (or third) reflected light beam R2 (R3) from the bottom surface even if the thickness of the substrate is small. However, as the width of the slit beam is reduced, the dynamic range of a detection signal (focusing signal) detected through, for example, synchronous demodulation also becomes narrow. With a narrow dynamic range, if the surface of the substrate is positioned considerably offset from the target (or ideal) position, the height of the substrate surface cannot be detected because it is positioned out of a detectable range. If this is the case, a so-called search operation must be performed for bringing the substrate surface within a detectable range of the auto-focus system while adjusting the height of the substrate. The search operation takes time to detect the height of the substrate surface, which causes the throughput of the exposure apparatus to drop.
This invention was conceived in view of the problems in the conventional technique, and it is an object of the invention to provide a surface height detecting apparatus that is capable of measuring the substrate surface height reliably and quickly and to provide an exposure apparatus using the height detecting apparatus.
To achieve the above and other objects, in one aspect of the invention, a surface height detecting apparatus has an illumination device for emitting detection light toward the surface of a substrate from an oblique direction with respect to the surface of the substrate. The profile of the detection light is shaped by an optical shaper into a slit-like profile, thereby generating a slit beam. An alteration device alters the width of the slit beam by controlling the optical shaper. A control oscillation mirror guides the slit beam onto the surface of the substrate, and vibrates the light beam reflected from the surface of the substrate in a direction parallel to the width of the slit to generate an oscillating slit beam. A detector detects the surface height of the substrate based on the oscillating slit beam.
The alteration device alters the width of the slit beam according to the thickness of the substrate. For example, as the thickness of the substrate becomes larger, the alteration device increases the width of the slit beam along the optical axis. The alteration device also increases the width of the slit beam when roughly detecting the surface height of the surface, and decreases the width of the slit beam when precisely detecting the surface height of the substrate. The width of the slit beam may also be adjusted according to the material of the substrate.
The surface height detector preferably includes a slit plate, which is positioned at an image-forming spot of the slit beam that was reflected by the surface of the substrate, and an amplitude controller for controlling the amplitude of the slit beam according to the width of the slit beam altered by the alteration device. The detector detects the height of the surface of the substrate based on the slit beam that has passed through the slit plate.
In another aspect of the invention, an exposure apparatus is provided that emits an exposure light beam to a mask on which a predetermined pattern is formed and transfers an image of the pattern through a projection optical system onto a photosensitive substrate surface. The exposure apparatus includes an illumination device for emitting detection light toward the surface of the photosensitive substrate from an oblique direction with respect to the surface, an optical shaper for shaping the profile of the detection light into a slit-like shape to generate a slit beam, and an alteration device for changing the width of the slit beam by controlling the optical shaper. A controlled oscillation mirror guides the slit beam onto the surface of the photosensitive substrate and oscillates the reflected slit beam along the width direction of the slit to generate an oscillating slit beam. The exposure apparatus also includes a detector for detecting the surface height of the photosensitive substrate based on the oscillating slit beam, and a driving device for driving the photosensitive substrate along the optical axis of the projection optical system based on the detection result of the detector so that the surface of the photosensitive substrate comes into alignment with the focusing position of the exposure light beam.
The width of the slit beam is preferably adjusted according to the thickness of the photosensitive substrate. The alteration device may be adapted to increase the width of the slit beam when roughly detecting the height of the photosensitive substrate and decrease the width of the slit beam when precisely detecting the height of the photosensitive substrate.
In operation, for example, after a photosensitive substrate is loaded on a stage and AF sensors are calibrated, information about the thickness, material, and the like of the photosensitive substrate is supplied. Based on the information, an appropriate width of the slit beam is calculated. The appropriate width is a width such that a reflected light beam from the surface of the substrate can be sufficiently separated from a light beam reflected from the bottom of the substrate. If the substrate is thin, the width of the slit beam is decreased. If the substrate is thick, the width of the slit beam is increased. Although a narrow width of the slit beam makes the dynamic range of the detector narrower, the slit beam reflected by the top surface of the substrate can be reliably separated from the light beam reflected by the bottom of the substrate. In addition, measuring accuracy is improved. On the other hand, if the width of the slit beam is increased, the dynamic range of the detector and the detectable range are broadened.
If the top surface of the substrate is coated with a metal layer so that incident light does not reach the bottom face of the substrate, it is not necessary to consider separation of the incident light components between a light beam reflected by the top surface and one reflected from the bottom face of the substrate. In this case, the width of the slit beam can be increased regardless of the thickness of the substrate to avoid unreasonably narrowing the detectable range for the surface height of the substrate. With the structure according to the invention, even the first substrate in a lot, which has a relatively large loading error, can be reliably detected.
An appropriate amplitude (oscillating amount) of the slit beam is determined based on the width of the slit beam, which is determined in the above-mentioned manner. The amplitude of the slit beam is adjusted by controlling, for example, a mirror that reflects the slit beam. It is preferable to set the amplitude of the slit beam to about double the width of the slit beam itself.